Component C4 interacts with C1 and C2 to form C3 convertase of the classic activation pathway. Systemic lupus erythematosus (SLE) is often associated with congenital C4 deficiency. Reduced levels of components of the classic pathway (C1, C2, C4, C3) are common after such activation, e.g. in SLE, acute serum sickness and conditions associated with circulating immune complexes. C4 has a mol wt of 210 kDa. The common form present in plasma and used in diagnostics assays is the C4c form offered here.